1. Field of the Invention
The present invention relates to an optical scanning apparatus and, more particularly, to an optical scanning apparatus which is used for a recording engine unit in a laser printer or the like to form an image by scanning a photosensitive member with a light beam.
2. Description of the Related Art
An optical scanning apparatus in an electrophotographic image forming apparatus drives a semiconductor laser in accordance with input image data and forms an electrostatic latent image corresponding to the image data on a photosensitive member.
A collimating lens converts laser light emitted from a semiconductor laser used as a light source for an optical scanning apparatus into almost parallel light, and a deflecting member such as a rotating polygon mirror deflects the light with a predetermined beam diameter. The light is then subjected to the focusing effect of an f-θ lens. At the same time, the f-θ lens corrects the distortion of the light so as to guarantee the temporal linearity of scanning. As a consequence, the laser light passing through the f-θ lens is imaged/scanned on the photosensitive member at a constant speed in the main scanning direction (the axial direction of the photosensitive member). Since the shift of a focal position on the photosensitive member is a factor that causes a deterioration in image quality, the optical scanning apparatus is designed to prevent the focal position of laser light on the photosensitive member from greatly shifting. For example, the dimensional accuracy and positioning accuracy of optical and mechanical components are improved, and the linear expansion coefficients of the components are optimized.
There is proposed a technique for changing the focal position of laser light on a photosensitive member by mechanically moving a collimating lens (see Japanese Patent Laid-Open No. 2-293877). There is also proposed a technique for changing the convergence of laser light by placing an electro-optics crystal as a focus correction unit on the exit side of laser light from a collimating lens (see Japanese Patent Laid-Open No. 4-264420).
In addition, there is disclosed a technique for detecting the focus shift of a light beam using a photosensor, also detecting a change in environmental element which is a cause of the focus shift, and correcting the focus shift on the basis of the detected focus shift and the detected change in environmental element (see Japanese Patent Laid-Open No. 2-93559).
The method proposed in Japanese Patent Laid-Open No. 2-293877 requires a complicated mechanism for mechanically moving the collimating lens. In addition, the response speed is low because of mechanical movement, and hence it is difficult to change the focal position of a light beam at high speed. For this reason, it is hard to say that this method can be suitably applied to a high-speed device such as a laser printer.
The method proposed in Japanese Patent Laid-Open No. 4-264420 does not require any mechanical movable portion in the optical system, and performs electrical control with an electro-optics crystal. This allows operation with high response speed. This method, however, requires new components such as an electro-optics crystal in addition to the collimating lens. As a result, the number of required components increases, and the optical scanning apparatus increases in size.
Furthermore, the focal position of laser light greatly depends on the positioning accuracy of optical components. The expansion and contraction of components due to thermal expansion will cause shifts between the actual positions of optical components and the mathematically calculated positions. These causes make it difficult to improve the accuracy of the focal position of laser light in an entire scanning range in the main scanning direction.